Many different methods can be used for the analysis of gases. Among other things, infrared optical sensors have become established, which have relatively low cross sensitivities and are characterized by high reliability. This type of sensor is therefore used in both medical engineering, for example, for the analysis of gaseous anesthetics, CO2 or dinitrogen monoxide, and safety engineering for the detection of hydrocarbons, carbon dioxide, etc.
Infrared optical sensors are based on the evaluation of spectral changes, which are caused by interactions of gases being analyzed with a measuring light. The spectrum of the measuring light may change due to absorption processes or spontaneous emission may occur after an interaction. It is necessary in most applications to detect and/or evaluate changes in markedly narrower wavelength intervals from a relatively broad-band spectrum in the infrared range. Spectral selection is necessary in these cases.
Distinction is made among the infrared optical sensors between dispersing and nondispersing systems. Dispersing systems often contain classical grid structures and have the advantage of using a larger amount of spectral information due to a wavelength-dependent splitting, but, on the other hand, instruments with such devices are mostly very expensive, relatively large and cumbersome to handle. Even though nondispersing systems are mostly more robust and more compact, it is frequently impossible to go below a certain cost limit due to the use of relatively expensive interference filters, precisely when a larger number of detectors (>2) is used.
Moreover, additional lens systems are frequently used for focusing with both methods, and these lens systems make such sensors more expensive and less compact.
However, focusing frequently cannot be done away with. To make additional focusing elements dispensable, it is known that attempts have been made to design dispersing elements macroscopically such that they possess focusing and imaging properties. Spherical concave structures, whose surface is periodically microstructured, may be mentioned as examples of this. As a result, an optical element is obtained, which combines the imaging properties of a hollow mirror with the dispersing action of a reflection grid. However, such elements require a large amount of material for their manufacture, and are difficult and expensive to manufacture. In addition, a markedly longer ray path to the site of detection is frequently associated with the use of such reflectively operated elements, as a result of which the probability that the signal to be evaluated is distorted by effects unrelated to the analyte increases.